1. Field of the Invention
The present invention relates to a printed circuit board (PCB) having axially parallel via holes. More particularly, the present invention pertains to a PCB having axially parallel via holes, in which an outer ground via hole, acting as a ground, is formed around a via hole for intercircuit connection in the PCB, thereby minimizing the effect of noise caused by the via hole.
2. Description of the Prior Art
In accordance with the recent miniaturization and multifunctionalization of portable information devices, such as notebook computers, PDAs, and mobile phones, demand for highly integrated PCBs used in the portable information devices is growing. Accordingly, high integration of via holes is required for circuit connection between layers of the PCB.
FIG. 1 is a sectional view of a PCB, through which various types of via holes 11, 12, 13, 14 are formed.
As shown in FIG. 1, the via holes, which are formed through the PCB, are divided into inner via holes 11 for connecting circuits of internal layers, staggered via holes 12, which have a step-shaped circuit connection path, stacked via holes 13, in which a plurality of via holes are laminated, and through via holes 14 for interlayer connection according to the purpose and shape.
The via holes of the PCB cause noise, such as transfer noise or reflection noise, due to impedance mismatching of a circuit pattern or a device therewith.
The noise caused by the via holes matters little for a low-speed signal, but significantly affects an output signal when a bandwidth is expanded and a signal processing speed increases due to a rapid increase in data transmission.
Hence, a method of processing a via hole using a laser drill, and a method of designing a through via hole and a staggered or stacked via hole have been studied in order to reduce noise caused by the via holes of the PCB.
However, the method of processing the via hole using the laser drill is problematic in that it is difficult to process the via hole when the PCB is thick, like a multilayered PCB, even though integration is improved and noise is reduced a little.
Meanwhile, in the method of designing the staggered or stacked via hole as well as the through via hole, it is possible to reduce a thickness of the PCB, which is processed by the laser drill. However, this method is problematic in that the inductance of a copper layer, acting as a conductor for the via hole, increases with an increase in a frequency, and that noise is significantly increased due to the effect of capacitance.
Turning to FIG. 1, a via hole structure of a conventional PCB is problematic in that since a power supply layer or a signal transfer layer 20 is exposed to transfer noise caused by the via hole, the power or the signal transfer is distorted.